Environmental monitoring efforts and water quality assessment in particular would benefit from widely available and inexpensive chemical assays and sensor technologies. Gas and liquid chromatography methods, and their coupling to mass spectrometry, are currently standard methods suggested by the US environmental protection agency (EPA) for detection of chemical toxins in drinking water. While these methods are considered sufficiently sensitive and accurate, their use is mostly confined to laboratory settings due to their size, weight, power requirement, peripheral equipment, cost, and sample preparation steps. There is a need for detection techniques which are cost-effective, sensitive, and portable.
One approach towards widespread toxin detection is the miniaturization of traditional chromatography systems. While there have been efforts which significantly reduce size and weight, scaling down and integrating the essential system components remains a challenge. Much of the work is focused on implementation of an efficient stationary phase in microstructures, and in miniaturization of pressure sources, pumps, and valves.
An alternative approach to realizing low-cost and portable toxins detection is developing novel assays which have increased functionality, avoid complex sample preparation (e.g., labeling), and are compatible with inexpensive system architectures and sensitive detection methods. Fluorescence based detection is the most sensitive method for on-chip applications, but typically requires autofluorescent analytes (a property that is not possessed by most toxins of interest) or fluorescent labeling (e.g. using immunoassays).
Recently, several fluorescence-based detection methods based on isotachophoresis (ITP) have been proposed. In ITP, sample ions simultaneously focus and separate according to their electrophoretic mobilities between a leading electrolyte (LE) and trailing electrolytes (TE). This creates purified, high-concentration, adjacent zones electromigrating at a uniform velocity. However, previous work on ITP analysis often relied on ad hoc assay design. For example, it may be necessary to include a labeled marker species in the assay that has an effective mobility between the effective mobilities of two analytes of interest. In such cases, a priori knowledge of analyte properties (i.e., effective mobilities) is needed to select an appropriate marker species.
It would be an advance in the art to provide an assay that does not require such a priori knowledge of analyte properties.